Optical microscopy and Raman spectroscopy are two examples of optical techniques that are useful in various applications but which suffer from certain limitations as conventionally implemented. With respect to optical microscopes, such devices have numerous applications in both the physical sciences as well as in the life sciences. In semiconductor manufacturing for example, visible light microscopes are used for inspecting semiconductor wafers following many of the several hundred process steps employed to fabricate semiconductor devices. This optical wafer inspection technique has advantages over the use of electron microscopy. In particular, optical microscopy is a non-destructive technique in that it does not involve breaking valuable wafers. Also, optical microscopy does not involve evaporating coating onto the samples, or evacuating the sample chamber, both of which can be time consuming. Further, optical microscopes typically do not cost as much as electron microscopes, and the technical skill level required to operate optical microscopes to obtain high quality micrographs typically need not be as high as that required to operate electron microscopes.
Notwithstanding the advantages of optical microscopes relative to electron microscopes such as those described above, in recent years there has been a significant decline in the sale of optical microscopes. This is partially due to a decline in their utility for semiconductor research and manufacturing, where the minimum feature size for present day devices has decreased to less than 0.5 microns, and in some advanced chip designs to less than 0.1 microns. In particular, because the ability of visible light optical microscopes to discern useful information concerning features of 0.5 microns or less is marginal, electron microscopes have increasingly become the tool of choice in observing such features.
In view of these considerations, and since the resolution of an optical imaging system scales linearly with wavelength, it is desirable to design an optical microscope that utilizes light at shorter wavelengths than light within the visible spectrum. A number of techniques involving shorter-wavelength light have been considered, yet these techniques suffer from various disadvantages. For example, while an optical microscope employing light within the near ultraviolet range (approximately 200 nm<λ<400 nm) may provide some wavelength advantage over a visible light optical microscope, the difficulties of image display and aberrations in optical components may not justify that advantage.
Also for example, a number of ultraviolet microscopes have been designed for the “soft X-ray” region, particularly at a wavelength of 2.48 nm. This wavelength is useful because of reduced water absorption by biological specimens in the range 2.4-4.4 nm. The radiation source is the six-fold ionized Nitrogen atom, N VII. However, it is difficult energetically to dissociate Nitrogen and then form the N+6 ion in an electronically excited state. Indeed, to perform such a process and thereby generate light at the desired wavelength, complicated methods and equipment such as pinched plasma sources and high-powered pulsed lasers are necessary. Further, because the atmosphere substantially absorbs light at the above-mentioned wavelengths, optical microscopes utilizing light at such wavelengths typically must be designed so that the transmission of light occurs within a high vacuum. Implementation of a microscope in a manner such that light is transmitted within a high vacuum, however, can be challenging and costly.
As for Raman spectroscopy, which employs Raman scattering, such techniques have been used to measure the mechanical stress in thin films, various material substrates and, more particularly, semiconductors. The techniques yield information about phonon frequencies, energies of electronic states, impurity content, composition (e.g., SiGe films), doping levels, temperature and mechanical stress. Information regarding such characteristics can be of interest for a variety of reasons. For example, mechanical stress can adversely affect the functioning and reliability of microelectronic devices, micro-electromechanical systems (MEMS), and nanostructures. Stress in films can cause changes in electron or hole mobility, current leakage, dislocations near silicide lines, cracks in chips, fractures in MEMS, breaking of solder bumps, stress migration, etc.
Despite the many applications, the use of Raman spectroscopy has significant limitations, at least some of which are similar to those discussed above as pertaining optical microscopy. Such limitations are particularly problematic when using Raman spectroscopy to measure local stress in sub-micron features in semiconductor devices. Raman spectroscopy is performed by directing light toward a target and detecting Raman scattered light that is reflected by the target. Such Raman scattered light is inherently of low intensity by comparison with the intensity of the incident light directed toward the target, and yet the intensity of the received Raman scattered light signals is of great importance to the overall effectiveness of the spectroscopic measurements. In order to ensure that the Raman scattered light is of sufficient intensity, many conventional Raman spectrometers employ high-intensity laser light sources in the visible and near ultraviolet portions of the electromagnetic spectrum.
Although such high-intensity laser light sources generating light in the visible and near ultraviolet portions of the electromagnetic spectrum are effective for some applications, they are increasingly ineffective for performing Raman spectroscopy in relation to semiconductor devices and other devices/structures having small (e.g., sub-micron) features. To effectively probe increasingly small features in an accurate manner, it becomes desirable that the Raman spectrometer attain higher levels of resolution. Attainment of such higher levels of resolution is closely and directly tied, on several counts, to using light of shorter wavelengths/higher frequencies than the light provided by the aforementioned conventional light sources. First, the Raman shifted signal indicated by the Raman scattered light reflected off of a target is an “average” of the illuminated target volume, which corresponds to the size of the illuminated spot size or area, multiplied by the penetration depth of the light into the target. Since both the spot size and the penetration depth defining a target volume are strongly wavelength dependent (for example, in Silicon, the penetration into the film surface is 3000 nm at λ=633 nm, but only ˜6 nm for radiation at 244 nm), higher resolution naturally flows from the use of light with shorter wavelengths. The intensities of the Raman peaks are also strongly wavelength dependent.
While such conventional light sources generating light in the visible and near ultraviolet portions of the electromagnetic spectrum are increasingly inadequate for providing desired higher levels of resolution, the desirability of a high-intensity light source for performing Raman spectroscopy only becomes higher as the target volumes of interest (and correspondingly the light wavelengths of interest) decrease. More particularly, as the features being probed on a target become smaller, the amount of Raman scattered light that is received from the target becomes smaller given the same light source (e.g., a pin point probing volume returns a very low signal by comparison with a larger target volume), and integration times become longer. Due to this factor as well as the above-mentioned factors, Raman scattering efficiency is proportional to λ−4 (where λ is the excitation wavelength). While in general the resolution of imaging optics improves linearly with decreasing wavelength, this does not overcome the aforementioned issues (spot size varies with λ2).
Further complicating matters, light at wavelengths shorter than those of the near ultraviolet range (e.g., light at wavelengths of λ<185 nm or λ<190 nm) typically experiences much more intense absorption in air than light in the visible and near ultraviolet portions. Since it is impractical and/or costly to require that Raman spectrometry be performed within a vacuum or near vacuum, the aforementioned absorption of light at wavelengths shorter than those of the near ultraviolet range is yet an additional factor contributing to the desire for a high-intensity light source to produce this type of light. Yet laser light sources producing light at shorter wavelengths corresponding to the onset of the vacuum ultraviolet region and within the vacuum ultraviolet region, where the intensity of the light is sufficiently high to overcome the above complications, are generally lacking.
For at least these reasons, it would be advantageous if a new optical microscope and/or a new Raman spectrometer, and/or one or more associated imaging systems, and/or one or more related methods of performing optical microscopy and/or Raman spectrometry could be developed. In at least some embodiments, it would be particularly advantageous if such an improved microscope, Raman spectrometer, imaging system and/or method utilized light at one or more wavelengths that were shorter than those of the visible light spectrum, so as to allow for enhanced viewing or probing of small features. Further, in at least some embodiments, it would be particularly advantageous if such an improved microscope, Raman spectrometer, imaging system and/or method could be implemented without the need for extremely complicated or costly light sources, and/or could achieve successful operation even without the use of a high vacuum to facilitate the efficient transmission of light.